1. Field of the Invention
The present invention relates to a hydraulic driving system for driving, for example, an operating machine such as a hydraulic excavator, and especially relates to a hydraulic driving system having a closed circuit with a single rod hydraulic cylinder and a hydraulic fluid inlet-flow and outlet-flow control unit for the closed circuit annularly connected to each other.
2. Description of the Related Art
Recently, in an operating machine such as a hydraulic excavator, a hydraulic circuit referred to as the so-called closed circuit has been known. The closed circuit is configured to be connected annularly (in a closed circuit-shaped manner) so that hydraulic fluid is directly fed to a single rod hydraulic cylinder as a hydraulic actuator from a hydraulic pump as a pressure generation source, and the hydraulic fluid after driving the single rod hydraulic cylinder and performing a predetermined work is directly returned to the single rod hydraulic cylinder. On the other hand, in contrast to the closed circuit, a hydraulic circuit referred to as the so-called open circuit has been also known. The open circuit is configured in such a manner that the hydraulic fluid is fed from the hydraulic pump to the single rod hydraulic cylinder through a throttle by a control valve, and the hydraulic fluid (return hydraulic oil) led to flow out of the single rod hydraulic cylinder is discharged to a hydraulic fluid tank. In comparison with the hydraulic circuit employing an open circuit method, the hydraulic circuit employing a closed circuit method is excellent in fuel economy performance because there is a little pressure loss caused by the throttle and energy of the return hydraulic fluid from the single rod hydraulic cylinder is regenerated by the hydraulic pump.
International Publication WO 2005/024246 discloses a related art with these kinds of closed circuits combined with each other. In International Publication WO 2005/024246, a first closed circuit with the hydraulic pump connected in a closed circuit-shaped manner is installed with respect to a boom cylinder as the single rod hydraulic cylinder, and also a second closed circuit with a hydraulic pump connected in a closed circuit-shaped manner is installed with respect to an arm cylinder. Further, an open circuit with a hydraulic pump connected through a control valve is installed with respect to a bucket cylinder. A distribution circuit is divergingly provided in order to distribute the hydraulic fluid delivered from the hydraulic pump of the open circuit, from a side of the hydraulic pump provided closer to the control valve of the open circuit to the boom cylinder and the arm cylinder. A check valve for preventing a backward flow of the hydraulic fluid from the closed circuit to the open circuit is provided in the distribution circuit.
In the related art disclosed in the above-described International Publication WO 2005/024246, the hydraulic pump of the closed circuit and the hydraulic pump of the open circuit are respectively driven, and the backward flow of the hydraulic fluid from the open circuit to the closed circuit, caused when, for example, the boom cylinder is extended, is prevented by the check valve. However, if there is a pressure difference between a side of the open circuit and a side of the closed circuit, there is a possibility that smooth motion of the boom cylinder may be impaired because an inlet-flow rate of the hydraulic fluid into the boom cylinder is suddenly changed when the hydraulic fluid on the side of the open circuit and the hydraulic fluid of the side of the closed circuit are merged by the distribution circuit and are fed to the boom cylinder.